It's always your fault, not your users fault. Even if it's your users fault, they have the money and you want it, so you have to fix it.
Try to get some people that may be users and make them use the app in front of you. Ask nicely. I remember that someone offered a free coffee in exchange of the help.
The most important part is that you must be 3 foots away, with your mouth closed and your arms crossed behind your back. Don't try to help. Don't talk. Don't give advice.
Only watch!
Did I say that you MUST have your arms crossed?
Watch and see what they do and where they are stuck. When they are completely stuck talk about their problems. Watch a few, and try to fix the problems they have. Get more guinea pigs and iterate.
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In some internal tool for the university I had similar problems. For example, after the presentation page they didn't understand how to continue. So I added a yellow arrow that appears after 10 or 20 seconds, something like the Windows 95 arrow. It fixed the problem. It's not very elegant, but they can't escape :). But it was important that they can complete the task without help.